The present invention relates to a sputtering target of a metal, such as copper or others, which can be easily oxidized, and a method for fabricating the sputtering target.
As interconnection layers of the current highly integrated semiconductor devices, copper interconnection layers have taken the place of aluminum interconnection layers.
Generally, electroplating is widely used for forming copper interconnection layers. In forming copper interconnection layers by electroplating, a base conductor layer of a metal, such as copper or others, must be formed in advance. To form such base conductor layer, generally sputtering is used.
FIG. 5A is a diagrammatic view of a magnetron sputtering system generally used in forming the above-described base conductor layer of copper or others. As shown, a substrate 104 of silicon or others to be sputtering processed is held substantially horizontal on a stage 102 disposed in a chamber 100. A heater (not shown) for heating the substrate 104 as required is provided in the stage 102. A sputtering target 106 is mounted on the inside of the chamber 100 above the substrate 104, opposed to the surface of the substrate 104. A shielding plate 108 is disposed surrounding the space between the substrate 104 and the sputtering target 106. A direct current source 110 interconnects the sputtering target 106 as a cathode and the substrate 104, so that a high negative voltage can be applied to the sputtering target.
In the sputtering processing, an internal pressure of the chamber 100 is decreased to a prescribed pressure, and a sputtering gas, such as argon gas, is fed into the chamber 100. Then, a voltage is applied by the direct current source 110. Thus, argon plasmas are generated between the sputtering target 106 and the substrate 104. The thus dissociated argon ions impinge on the sputtering target 106, which is the cathode to sputter the metal atoms. The sputtered metal atoms arrive at the substrate 104 to thereby form a metal layer on the substrate 104.
To form the base conductor layer of copper by sputtering, a sputtering target of copper is necessary. In comparison with the sputtering target of aluminum, the surface of the copper sputtering target is very easily oxidized. That is, as shown in FIG. 5B, usually an oxide layer 112 of copper oxide is formed on the surface of the copper sputtering target 106.
When copper is applied to a substrate of silicon or others by the magnetron sputtering system by using a copper sputtering target with an oxide layer thus formed on the surface, it is necessary to prevent the oxide on the surface of the target from adhering to the substrate. To this end, conventionally, a prescribed amount of sputtering processing is performed on a dummy substrate to thereby sufficiently remove the oxide layer on the target surface, and sputtering processing is performed on a substrate to apply copper thereto. The processing for removing the oxide layer formed on the sputtering target surface before said sputtering processing is called burn-in processing or pre-sputtering processing.
In using the conventional copper sputtering target with an oxide layer formed on the surface, a disadvantage is that particles tend to adhere to a substrate to be sputtering processed. The adhesion of particles to a substrate in using the copper sputtering target will be explained with reference to FIGS. 6A and 6B. FIGS. 6A and 6B are diagrammatic views of states of the shielding plate of the sputtering system after the burn-in processing and the sputtering processing.
In the burn-in processing, the oxide from an oxide layer on the surface of the copper sputtering target adheres to the surface of the shielding plate in the chamber to form a contaminant layer of the copper oxide. Then, in the intended sputtering process, the copper adheres to the contaminant layer formed on the surface of the shielding plate. FIG. 6A is a diagrammatic view of the contaminant layer 114 formed on the surface of the shielding plate 108 and the copper layer 116 formed on the contaminant layer 114.
Generally, in the sputtering processing, depositing particles impinge at high energies on an object for the particles to adhere to, and it can be said that the formed deposited layer does not easily peel off. However, in the above-described burn-in processing, the contaminant layer 114 formed on the surface of the shielding plate 108 is formed of the oxide called copper oxide, which is exhibits poor adhesion to the surface of the shielding plate 108 of stainless steel or others. Then, as shown in FIG. 6B, the contaminant layer 114 easily peels off the surface of the shielding plate 108 due to stresses of a copper layer 116 formed on the contaminant layer 114 or thermal stresses caused by the sputtering process. The deposits, which have grown on the surface of the shielding plate 108, peel off, with the result that the particles adhere to a substrate in the sputtering processing.
The influence of the above-described particle adhesion to substrates on semiconductor devices is becoming an unignorable and critical problem as the high integration goes on.
As a method for preventing the surface of the sputtering target from being oxidized, an adhesive tape or the like is adhered to the surface until a time immediately before the sputtering target is mounted on the sputtering system. This method is effective in shielding the sputtering target surface from the atmospheric air to prevent the oxidation. However, the deposited layer formed by using the sputtering target with an adhesive tape or the like applied thereto is contaminated with organic substances. Because of the sputtering target surface contaminated with an organic substance, a contaminant layer is formed between the shielding plate and the deposited layer, and resultantly the deposited layer on the shielding plate surface tends to easily peel off. As a result, in the sputtering processing, particles tend to adhere to a substrate due to the release of the deposits on the shielding plate surface.